TPS adjustment procedure

[Tom Drew]

Hello,

I am continuing to have rich running issues with my '72 280SE 4.5. I've gotten a wealth of info from this forum already, and after having checked, replaced and/or adjusted the various components of the FI system, I'm down to the 2 last components I need help with: TPS switch & MPS.

Current status of the engine is this: Steady vacuum of about 17.25 @ idle. Starts instantly and idles smoothly. Good power throughout the whole range, no flat spots or stumbles. Excessive fuel consumption, black sooty plugs and black sooty tailpipe. Valve stem seals replaced & valves adjusted. Difficult hot start if tried to start 5 min after shutdown. Let the car sit for 30 minutes and it will start. Car has Pertronix ignition. Have verified/replaced the following: Air temp sensor, water temp sensor, cold start valve, idle air valve, fuel pressure regulator & damper (pressure set @ 28psi), throttle linkage, trigger points, continuity and resistance checks of the harness & ECU. Vacuum system tight with no leaks. I have all the relevant MB service manuals and read and use them.

I am left with these possibilities:

1) TPS sending a continuous full rich signal to the ECU.

I can't discern how to properly adjust the TPS from the manual as the only procedure they give is with the use of the Bosch tester. Can some one give me the procedure using a DVM for testing - I understand how to physically adjust the switch, but I need to know what terminals I need to measure with the meter and what values I am looking for.

2) Internal problem with the ECU causing a continuous full rich condition.

It seems that from all available information, it is essentially impossible to test the ECU in any sort of comprehensive way without the original Bosch tester. Does anyone have one of these that would be willing to test ECU's for us? It is hard to believe that none of these units exist any more. I'm not especially comfortable with the "use a known good ECU" approach.

3) MPS adjustment to lean out mixture.

Lots of information and opinions on adjusting the MPS on the forum. After reading Paul B. Anders research on the MPS:

http://members.rennlist.com/pbanders/manifold_pressure_sensor.htm

When we are adjusting the 4mm hex screw on the MPS, are we adjusting the full load stop? Does the 4mm hex screw screw all the way out to reveal other adjusting screws, as Anders shows on the 914 VW MPS's he is picturing? Do the 914 VW and M117 MPS's have the same adjusments?


Any help and advice would be greatly appreciated. I'm aware of Barri and others that have successfully gone the Megasquirt route, and would probably do this except for the time commitment involved. I'm in a very busy phase of life with job & kids and my available time to wrench is precious little.

Thanks,

Tom

[cth350]

The unit on the throttle isn't a TPS like anything conventional. It's just a multi-position switch that either works or doesn't. You can adjust it's relative position on the throttle shaft to control when it gets its first click, but the thing does nothing more than send a series of 10 or so on-off transitions as you open the throttle.

I have the bosch tester, it's a nice-to-have. But a multimeter and the instructions in the service manual is really all you need, once you stick a CO% meter up the tailpipe.

Correcting a rich running condition really needs that sniffer, because you can't tell if small things you are doing are having any impact. For instance turning the adjustment wheel on the side of the box.

-CTH

[Gurunutkins]

Just so you have it here are the instructions. I found if it was not set correctly at "0" for idle mine tended to run quite rich when i still used the original D-jet system.
Here is a pic of the insides of the throttle valve (the djet R107 one is in the middle) it has a resting point and then the 10 squigles which the ecu counts as you accelerate and then a "break" on the switch itself which puts it to "0" reading at WOT so you can get a signal for the ecu to read as flooded when starting the car and the MAP sensor is at cranking pressure.



here is the testing set up using a multimeter to test the resting stages. this is for testing at the ecu plug so just pull the plug at the TPS and use your multimeter to bridge the relevant pins and when you get the readings just firm up the screws again.



and here is to test the moving stage



at WOT the reading at pins 47/12 and 14/2 should go back to "0"

the little screws are a real PITA to get at and whatever you do make sure the throttle is fully closed at idle before you start! remember all of the idle air goes through the auxilliary air valve, no air through the throttle butterfly until you touch the accelerator!

cheers
Barri

[Graham]

Originally Posted by Gurunutkins

Just so you have it here are the instructions. I found if it was not set correctly at "0" for idle mine tended to run quite rich when i still used the original D-jet system.
Here is a pic of the insides of the throttle valve (the djet R107 one is in the middle) it has a resting point and then the 10 squigles which the ecu counts as you accelerate and then a "break" on the switch itself which puts it to "0" reading at WOT so you can get a signal for the ecu to read as flooded when starting the car and the MAP sensor is at cranking pressure.



here is the testing set up using a multimeter to test the resting stages. this is for testing at the ecu plug so just pull the plug at the TPS and use your multimeter to bridge the relevant pins and when you get the readings just firm up the screws again.



and here is to test the moving stage



at WOT the reading at pins 47/12 and 14/2 should go back to "0"

the little screws are a real PITA to get at and whatever you do make sure the throttle is fully closed at idle before you start! remember all of the idle air goes through the auxilliary air valve, no air through the throttle butterfly until you touch the accelerator!

cheers
Barri

Barri - The pictures are not showing up in this post.

I have checked my TPS and it reads infinity at idle position. Thinking this is probably one of causes of rich running and why many of us have to pump the accelerator during hot starts.

My problem at moment, is to get at the bottom screw on the switch - It wont move with just top one loosened. May be easier to remove the throttle housing, but I don't have a gasket!

ADDED: Decided there is no way to loosen the bottom screw. Took the housing off. Only lost one nut down in VEE TPS is not working at all. It reads infinity except at wide open. Tempted to put it back with old gasket - hard to see it is even there! But will probably get one if they can get it here overnight.